Using alpha channel of generated images for cycles
I'm not sure I understand the question entirely..

If you want an image with a transparent background,
Make sure you set the image background to completely transparent (assuming that is what you want)
  

When you save the image you also want to make sure you use an image format that supports alpha channels - png would be the most common.

The RGBA button enables saving the alpha channel and is only visible if the image format supports alpha.

(If you want to render the image, the RGBA button is in the render settings panel.).


You can then set up a simple material such as this: